Direct Access Arrangement (DAA) circuitry may be used to terminate the telephone connections at a phone line user's end to provide a communication path for signals to and from the phone lines. DAA circuitry includes the necessary circuitry to terminate the telephone connections at the user's end and may include, for example, an isolation barrier, DC termination circuitry, AC termination circuitry, ring detection circuitry, and processing circuitry that provides a communication path for signals to and from the phone lines. Examples of DAA circuitry known in the art may be found described in U.S. Pat. No. 6,385,235 and in U.S. patent application Ser. No. 09/347,688 filed Jan. 2, 1999 and entitled “DIGITAL ACCESS ARRANGEMENT CIRCUITRY AND METHOD HAVING A SYNTHESIZED RINGER IMPEDANCE FOR CONNECTING TO PHONE LINES” by Tuttle et al., the disclosure of each being incorporated herein by reference.
Generally, governmental regulations specify the telephone interface requirements and specifications for a variety of parameters including AC termination, DC termination, ringer impedance, ringer threshold, etc. For example, Federal Communications Commission (FCC) Part 68 governs the interface requirements for telephones in the United States. However, the interface requirements world wide are not standardized, and thus, in countries other than the United States the applicable standards may include the TBR21, NET4, JATE, and various country specific PTT specifications. Because the interface requirements are not standardized from country to country, often different DAA circuitry is required for use in each country in order to comply with the appropriate standard. The requirement for different DAA circuitry, however, limits the use of one phone line interface in a variety of countries. Thus, for example, a modem in a laptop computer configured for interfacing with a phone line in one country may not necessarily operate properly in another country. Further, the requirement for different DAA circuitry in various countries hinders the design of a single integrated cost effective DAA solution for use world wide.
As mentioned above, the telephone interface requirements generally include specifications for DC termination of the telephone line. For example, the DC impedance that the DAA circuitry presents to the telephone line (typically≦300 Ω) may be required by regulations to be less than the AC impedance that the DAA circuitry presents to the telephone line (typically ≈600 Ω). Consequently, inductive behavior is required from the section of the DAA circuitry that sinks DC loop current, which is typically called the DC termination or DC holding circuitry. This inductive behavior of the DC holding circuitry should provide both high impedance and low distortion for voiceband signals. The DC termination specifications may also include limits for the maximum current and power dissipation. For example, the TBR-21 specification requires the DC holding circuit to limit DC current to less than 60 mA with a maximum power dissipation of approximately 2 watts. Examples of DC holding circuitry known in the art may be found described in U.S. Pat. No. 6,201,865, the disclosure of which is incorporated herein by reference. The design of a DC holding circuit for use with multiple standards may be complicated in that the various international specifications may conflict with regards to off-hook settling times and pulse dialing templates (which may require fast settling time constants) and high speed interface designs (such as for use in modems) which require very low frequency operation (i.e. approximately as low as 10 Hz). Furthermore, it is desirable to implement such DC holding circuits in a manner that does not cause excessive distortion at low and high frequencies.
In order to pass homologation for many European countries, DAA circuitry must pass a “fast transient” test. In such a test, the tip and ring are capacitively couple to a signal line that has very fast switching transients at high voltage. To pass the test, the modem should not drop the connection in the presence of this interference. During a fast transient test, the DC holding circuit of the DAA circuitry may be disturbed by the fast switching transients. It may require a relatively long time (e.g.; seconds) to resettle the DC holding circuitry, resulting in dropped modem connections. This may be illustrated with reference to FIGS. 4 and 5.
FIG. 4 is a simplified schematic that illustrates components of a prior art fast transient network 400 that employs only one stage of fast transient filtering, and shows a fast transient filter circuit block 410 and DC holding current (IDCHOLD). The fast transient network components illustrated in FIG. 4 include capacitor C41, resistors R41 and R42, and op amp OA41. As shown in FIG. 4, voltage V1 is present at the line side node between fast transient filter circuit block 410 and resistor R42, voltage V2 is present at the node between fast transient filter circuit block 410 and capacitor C41, and voltage V3 is present at the node between capacitor C41, resistor R41 and op amp OA41. For the simplified schematic of FIG. 4, the relationship between voltages V1, V2 and V3, and DC holding current (IDCHOLD) may be approximated using the following relationships:V2=V1(1/(1+sτ1));V3=V2[sτ2/(1+sτ2)];andIDCHOLD=V1/R42[1−(1/(1+sτ1))(sτ2/(1+sτ2))]where:τ1=Fast Transient Filter≈1/2π(40 khz)τ2=DC Holding Frequency≈1/2π(1 hz)
FIG. 5 illustrates the prior art relationship between DC holding current and frequency for the simplified fast transient network of FIG. 4 that employs only one stage of fast transient filtering. As shown in FIG. 5, the error in DC hold at high frequencies may be sufficient to cause the DAA to fail AC termination tests for return loss in some countries.